Will 1998 Lincoln Town Car seats fit a 1990 Car?

It's highly unlikely that 1998 Lincoln Town Car seats will fit a 1990 Lincoln Town Car (or any other make/model car from that era). While both are Town Cars, there were significant design changes between those model years. The seat mounting points, frame dimensions, and even the seatbelt systems are likely to be different and incompatible. You'd need to compare the actual seat frames and mounting points very carefully to even have a chance, and that's still a long shot.